Prior art attempts at maintaining constant D.C. voltage output from a D.C. power supply frequently make use of phase controlled or SCR bridge rectifiers which are triggered into conduction in accordance with the value of the A.C. mains voltage input to be rectified. Such phase controlled bridge rectifiers are generally capable of generating D.C. voltage outputs at two or three times the desired level, thus providing the reserve capacity necessary to compensate for A.C. mains variations. Under normal operating conditions, however, relatively large firing delays are required to achieve proper conduction in the phase control SCR's employed by the bridge rectifiers. Large SCR firing delays, of course, result in poor power factors as well as high RMS currents through the SCR's.
In an effort to overcome the disadvantages associated with large SCR firing delays, prior art bridge rectifier circuits have utilized a second, short-time rated phase controlled bridge rectifier which is connected in parallel to the primary bridge rectifier and activated in response to A.C. mains voltage drops. FIG. 1 illustrates a prior art circuit configuration of this type, having parallel-connected primary and auxiliary bridge rectifiers 2 and 4. A transformer 6 taps the incoming A.C. mains line 7 to provide power for the auxiliary bridge, and the auxiliary bridge SCR's 8 are only triggered into conduction for those intervals during which the A.C. mains voltage input dips below a threshold level. Supplemental voltage furnished by the auxiliary bridge rectifier in the FIG. 1 circuit configuration adequately compensates for A.C. mains voltage variation while simultaneously permitting the more efficient use of relatively low voltage SCR's 9 in the primary bridge rectifier. Despite the advantages inherent in performing voltage regulating functions with paired bridge rectifiers, however, the need for high voltage SCR's in the auxiliary bridge of the FIG. 1 circuit configuration is not eliminated. The parallel nature of the connection between the auxiliary and primary bridge rectifiers results in the application of full A.C. mains voltage across the primary of the transformer 6 interconnecting the two bridges. In conventional phase controlled, parallel-connected bridge rectifier circuits, the step-up ratio of transformer 6 is typically two-to-one. Hence, the secondary voltage of the transformer can rise to twice the value of the A.C. mains voltage under normal operating conditions, and again the use of high voltage SCR's, this time in the auxiliary bridge, is required. Because low current, high voltage SCR's are not generally available, it would be of obvious benefit to provide a bridge rectifier configuration capable of using low voltage rated silicon controlled rectifiers to provide a constant D.C. voltage output regardless of variations in the bridge rectifier A.C. mains voltage input.